The present invention relates to a low friction servo controlled precision linear motion drive apparatus and in particular, a precision linear motion drive which converts rotary motion into linear motion and for use in apparatus for image recording, scanning, or measuring.
In certain image scanning devices, an image is scanned in two dimensions by a linear motion system traveling linearly along a slow scan axis of an imaging device, while a scanning device scans the image along a fast scan axis perpendicular to the slow scan axis. Such an imaging system requires precise synchronization between the slow scan axis motion and fast scan axis scanning and may also require that the synchronization be maintained for both forward and backward slow scan travel. It is necessary to minimize positional errors which result in imaging errors due to slippage or backlash of the drive elements and mechanical resonance which may occur during scanning. In addition, velocity errors due to various electrical and mechanical system resonance, slippage, or friction and drag variations, must also be minimized. Furthermore, since an image scanning linear motion system travels on an axis parallel to the slow scan axis of the image to be scanned, as well as perpendicular to the fast scan axis, it is advantageous for the axis of linear motion to be accurate and straight.
An example of a scanning mechanism of this type, disclosed in U.S. Pat. No. 1,746,407, has a cylindrical inner surface on which light is scanned upon by a beam deflector mounted on a rotating shaft. The cylindrical surface is moved linearly in a slide type of guide relative to the beam deflector mounted on the rotating shaft. This type of linear drive requires an operating space twice the length of the cylinder, due to the cylinder being driven linearly relative to the beam deflector.
Another linear motion drive apparatus is disclosed in U.K. Patent No. 1,185,115, in which a carriage is moved linearly inside of a cylindrical drum by a rotating screw engaging a nut assembled in the carriage. The drive requires few parts as the carriage is guided by a longitudinal slot in the cylinder, however the slot limits the usable scanning area. The screw is rotated by a motor mounted outside of the cylinder. Because of the precise accuracy demanded in image scanning apparatuses, the lead screw of such a system must be machined with particular tolerances, proving to be a costly process. The mass, inertia, and high friction of the lead screw require additional motor power to be driven at the required speeds, which further increases costs.
Another embodiment shown in the same reference uses a belt connected to the carriage to pull the carriage linearly inside the length of the cylinder. The belt is driven by means of pulleys mounted externally of the cylinder at the two ends of the cylinder. The pulleys are driven by an auxiliary motor. The carriage is supported by guide rollers which are in contact with the inner surface of the cylinder and which center the axis of linear motion with the axis of the cylinder. A longitudinal guide is provided to prevent the carriage from rotating relative to the cylinder. Both of these embodiments have motors and other elements of the drive systems mounted externally of the cylinder, requiring valuable space within the imaging system.
Accordingly, it is an object of the present invention to provide a linear motion drive apparatus mounted on a carriage of a linear transport system, thereby propelling the carriage independently of external drive elements and requiring minimal operating space substantially equal to the length of the slow scan axis.
Yet, another object of the present invention is to provide a friction drive means on a slow scan axis of travel and to control the drive means with a precision servo feedback system.